An MRI apparatus irradiates RF signals that are electromagnetic waves to an object disposed in a homogeneous static magnetic field generated by static magnetic field magnets, excites nuclear spin in the object, receives NMR signals that are electromagnetic waves to be generated by the nuclear spin, and performs signal processing in order to image the object. The RF signal irradiation and the NMR signal reception are performed by an antenna or an antenna device referred to as an RF coil (hereinafter, referred to as an RF antenna) that transmits and receives radio-frequency electromagnetic waves.
Since RF signals are irradiated to an object, the MRI apparatus needs to be controlled so as not to increase the object temperature due to the RF signals and get burned. Therefore, a SAR that is a human body absorption rate of radio waves (specific absorption rate) is managed strictly and accurately. This is referred to as SAR management. In an MRI apparatus of three teslas or more, normally, the SAR management is made by monitoring irradiation power of the RF signals in real time using a SAR monitor.
Irradiation power (Pinput) of RF signals to be input to the RF antenna is represented by the sum of that causes heat generation of the RF antenna due to consumption in the RF antenna (power consumption by the antenna: Pantenna) and that causes heat generation of an object due to consumption in the object (power consumption by the object: Pobject).Pinput=Pantenna+Pobject  (1)
In order to perform accurate SAR management, it is required to grasp the power consumption by the object Pobject. The power consumption by the object Pobject can be calculated using, for example, Q-factors of resonance of the RF antenna. That is, a Q-factor (Qempty) in a state where the object (patient) is not inside the RF antenna and a Q-factor (Qloaded) in a state where the object is inside the RF antenna are measured and obtained, and the values are used for calculation by the following equation (2) (for example, refer to Patent Literature 1).Pobject=Pinput×(1−Qloaded/Qempty)  (2)